Special attention will be given to the aldolases and hydratases and in particular to the ability of enzymes to expel a halogen substituent during the reaction. Mixed function oxygenases responsible for the initiation of certain aromatic degradative pathways will also be investigated in the purified condition. The reactions by which P. putida and Acinetobacter degrade p-hydroxyphenyl acetic acid to succinic semialdehyde and pyruvate have been elucidated, new metabolites have been discovered and an aldolase that functions in the sequence has been purified. A mixed function oxygenase from P. acidovorans has been partially purified and its stoichiometry established. This FAD-requiring enzyme catalyzes hydroxylation of p-hydroxyphenylacetic acid, giving homogentisic acid with migration of the side-chain substituent on the nucleus.